There is a push to operate natural gas engines at leaner air/fuel ratios to achieve higher power density, improved thermal efficiency, and low oxides of nitrogen (NOx) emissions. As mixtures become leaner the burden on ignition systems rises. Prechamber spark plug technology is an effective way to provide reliable ignition even with homogeneous lean mixtures in the main cylinder. Reliable combustion in the prechamber plug generates the necessary pressure to drive turbulent jet combustion in the main combustion chamber. However, such systems have limitations beyond which the air/fuel ratio is too lean to yield reliable combustion in the prechamber and therefore in the main chamber as well. To overcome this limitation, a number of manufacturers have produced (and are using) fuel-fed prechambers (fuel FPCs, also known as scavenging or enriched prechambers), where a richer air/fuel ratio is created within the prechamber by the precise delivery of a small amount of fuel to the prechamber. Typically a fuel FPC has considerably more volume and thus can deliver much more energic jets than a prechamber spark plug. This richer mixture in the prechamber creates reliable ignition and combustion, which in turn generates the rapid pressure rise to drive turbulent jets into the main chamber.
However, fuel FPCs in internal combustion engines generally produce a significant portion of the total NOx output of an engine. This is due to the combination of time, temperature and the presence of nitrogen in the air used to combust the fuel in the prechamber. On the other hand, reliable ignition of the very lean mixtures in the main cylinder would be very difficult without these high energy jets expelled from the prechamber. The problem is achieving reliable combustion in the prechamber without significant NOx production.
A dual-fuel engine is an engine that is configured to run on two different types of fuel. For example, a dual-fuel engine can run on both natural gas and diesel. The dual-fuel engine can run on a single fuel as the primary fuel or a mixture of the two fuels. However, dual-fuel engines can have issues that limit efficiency. For example, engines that operate with spark ignition on natural gas do not operate at the same compression ratios as diesel engines primarily because of the issue of knock. These engines have lower compression ratios than diesel engines to prevent knock. Thus, a dual-fuel engine having a lower compression ratio for natural gas operation will have reduced efficiency during diesel operation.
Like reference symbols in the various drawings indicate like elements.